Elimination of stitch artifacts in a planar illumination area

ABSTRACT

In one aspect, a planar illumination area includes two light-guide elements, each with an out-coupling region. At least a portion of each out-coupling region overlaps with at least a portion of the other. The overlapping region emits a substantially uniform light output power.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of U.S. ProvisionalPatent Application No. 61/006,110, filed on Dec. 19, 2007; U.S.Provisional Patent Application No. 61/064,384, filed on Mar. 3, 2008;U.S. Provisional Patent Application No. 61/127,095, filed on May 9,2008; and U.S. Provisional Patent Application No. 61/059,932, filed onJun. 9, 2008. The entire disclosure of each of these applications isincorporated by reference herein.

TECHNICAL FIELD

In various embodiments, the invention relates to systems and methods forplanar illumination using discrete waveguide-based lighting elements.

BACKGROUND

Using a point light source, such as a light-emitting diode (LED), tocreate a planar, uniformly emitting illuminating surface is difficult.Complex optical structures are required to distribute the light emittedfrom the LED evenly over the entire illuminating surface. An example ofsuch a structure is a light guide that receives point-source light on anedge of the guide and distributes the light uniformly over a surface ofthe guide. As shown in FIG. 1, an edge-illuminated structure 100 may usea side-emitting point light source 102 that transmits light 104 to anedge 106 of a light guide 108. The light guide 108 distributes thetransmitted light 104 to a top surface 110. The light source 102 isseparate from the light guide 108.

The number of light sources that may illuminate the structure islimited, however, by the lengths of the light-guide edges and thedimensions of the light sources. As the surface area of the guideincreases, more light sources than can physically fit on the light-guideedges may be required to maintain a constant illumination on the surfaceof the guide, ultimately setting an upper bound on the surface area.Moreover, an edge-illuminated light guide requires side-emitting,pre-packaged light sources, thereby limiting the number and types oflight sources that may be utilized. Further, the structure required tocouple light from a side-emitting light source into an edge of the lightguide may impede miniaturization of the planar illumination system.

A planar illumination surface constructed from a plurality of lightguides may exhibit non-uniform light intensity at the borders betweenadjacent light guides, or “stitches.” For example, the edges of thelight guides may be non-uniform, allowing non-light-emitting gaps toform between the light guides. In addition, the direction of propagationof the light in two adjacent light guides may be different, creating anon-uniform pattern of light emission at the border. Finally, tiles thatoverlap one another may allow stray light to escape in the overlappingarea. Clearly, a need exists for a planar illumination surface that isassembled from a plurality of light-guide elements and that emits auniform light.

SUMMARY

Embodiments of the present invention prevent a non-uniform distributionof light from occurring at the borders between light-emitting elements.For example, the sidewalls of the light-guide elements may be polished,curved, or set at a 90-degree angle to either reflect or refract lightas desired. An index-matching material may be deposited between adjacentlight-guide elements and/or on the light-emitting surfaces oflight-guide elements. A light-absorbing surface may be placed in theregion where two light-guide elements overlap, and the light-guideelements may emit a changing distribution of light in the overlappingregion.

In general, each light-guide element is an integrated monolithic lightguide that includes spatially distinct in-coupling, concentration,propagation, and out-coupling regions. The in-coupling region collectsthe light emitted from the LED light source and the out-coupling regionemits light to create the planar illumination. A light source may beadjacent to the in-coupling region of the element, but need not bepositioned at its edges. The in-coupling region of one light-guideelement may be at least partially covered by the light-emitting regionof an adjacent element. In this manner, continuous illuminating surfacesof any desired size can be constructed by tiling the requisite number oflight-guide elements, since unlit areas of one element will be occludedby overlying lit areas of an adjacent element.

In general, in a first aspect, a planar illumination area includes afirst light-guide element including a first out-coupling region and asecond light-guide element including a second out-coupling region. Atleast a portion of the second out-coupling region overlaps at least aportion of the first out-coupling to define an overlapping region havingan area. The overlapping region emits a substantially uniform lightoutput power over the area.

One or more of the following features may be included. The light outputpower of the overlapping region may differ from the light output powerof the first and second out-coupling regions by no more than 10%. Thechange in light output power between the overlapping region and thefirst and second out-coupling regions may be gradual. The firstout-coupling region may include light-scattering elements therein andthe first and second out-coupling regions may overlap only partially. Adensity of the light-scattering elements outside the overlapping regionmay be greater than a density of the light-scattering elements insidethe overlapping region. The density of the light-scattering elementsinside the overlapping region may decrease along one direction.

The second out-coupling region may be transparent in the overlappingregion. Light emitted from the first out-coupling region may passthrough the second out-coupling region in the overlapping region. Thefirst and second out-coupling regions may overlap only partially, andthe planar illumination area may further include a third light-guideelement including a third out-coupling region. A portion of the thirdout-coupling region may overlap a second portion of the secondout-coupling region to define a second overlapping region having asecond area. The second overlapping region may emit a substantiallyuniform light output power over the second area.

The first light-guide element may include a non-vertical sidewall andthe second light-guide element may include a second, complementarynon-vertical sidewall within the overlapping region. A top surface ofthe planar illumination area may be substantially flat. Additionallight-guide elements may form, with the first and second light-guideelements, an array extending in first and second directions, and thelight-guide elements may overlap in the first direction and may notoverlap in the second direction, or may overlap in both the first andsecond directions. The overlapping region may include a light-absorbingmaterial.

In general, in another aspect, a method of forming a planar illuminationarea includes providing a first light-guide element including a firstout-coupling region. At least a portion of the first light-guide elementis overlapped with a second light-guide element including a secondout-coupling region, thereby forming an overlapping region including atleast a portion of the first out-coupling region and the secondout-coupling region. The overlapping region emits a substantiallyuniform light output power.

In general, in another aspect, a planar illumination area includes firstand second light-guide elements including first and second sidewalls,respectively. An index-matching material covers the first and secondsidewalls. The index of refraction of the index-matching material isapproximately equal to an index of refraction of at least one of thefirst light-guide element or the second light-guide element.

One or more of the following features may be included. The secondsidewall may be adjacent to the first sidewall and the index-matchingmaterials may be located between the first and second sidewalls. Thefirst and second light-guide elements may overlap and the index-matchingmaterial may cover the first and second light-guide elements to create asmooth surface thereover.

In general, in another aspect, a planar illumination area includes firstand second light-guide elements including first and second polishedsidewalls, respectively. The second polished sidewall is adjacent to thefirst polished sidewall. The second light-guide element receives lightfrom the first light-guide element through the first polished sidewalland the second polished sidewall. An index-matching material may belocated between the first and second sidewalls, and an index ofrefraction of the index-matching material may be approximately equal toan index of refraction of at least one of the first light-guide elementor the second light-guide element.

These and other objects, along with advantages and features of thepresent invention herein disclosed, will become more apparent throughreference to the following description, the accompanying drawings, andthe claims. Furthermore, it is to be understood that the features of thevarious embodiments described herein are not mutually exclusive and mayexist in various combinations and permutations.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings, like reference characters generally refer to the sameparts throughout the different views. In the following description,various embodiments of the present invention are described withreference to the following drawings, in which:

FIG. 1 is a schematic elevation of a prior-art structure with aside-emitting point light source in accordance with an embodiment of theinvention;

FIG. 2 is a schematic elevation of a structure with a Lambertian lightsource in accordance with an embodiment of the invention;

FIGS. 3 and 4 depict perspective and elevational views, respectively, ofan illustrative embodiment of a planar illumination unit;

FIG. 5 depicts graphs of exemplary properties of planar illuminationsystems;

FIG. 6 schematically depicts a concentration region and its behavior inaccordance with an embodiment of the invention;

FIGS. 7-11 are perspective view of a two-source, one-source, asymmetric,two-source folded, and single-source folded light-guide elements,respectively, in accordance with embodiments of the invention;

FIGS. 12-14 depict various views of segment-assembly planar illuminationareas in accordance with an embodiment of the invention;

FIG. 15 is a perspective view of a stripe planar-illumination unit inaccordance with embodiments of the invention;

FIGS. 16A and 16B depict perspective and plan views, respectively, of aplanar illumination area assembled from the illumination unit shown inFIG. 15;

FIGS. 17A and 17B depict perspective and plan views, respectively, of aplanar illumination area assembled from asymmetric stripe elements inaccordance with an embodiment of the invention;

FIG. 18 is a plan view of a planar illumination area formed from foldedtwo-source light-guide elements in accordance with an embodiment of theinvention;

FIG. 19 is a perspective view of a planar illumination area formed fromfolded asymmetric light-guide elements in accordance with an embodimentof the invention;

FIGS. 20-22 depict cross-sections of non-uniform light emitted by aplanar illumination area in accordance with embodiments of theinvention;

FIGS. 23 and 24 are elevational views of light-guide element sidewallstructures in accordance with embodiments of the invention;

FIG. 25 is a plan view of a planar illumination area with a transparentdiffusive sheet in accordance with embodiments of the invention;

FIG. 26 is a perspective view of a back-light unit (BLU) applicationwith tiled elements in accordance with an embodiment of the invention;

FIG. 27 is a perspective view of a planar illumination area covered by alight-absorbing surface in accordance with an embodiment of theinvention;

FIG. 28 depicts a cross-section of a planar illumination area with alight-absorbing surface placed in the regions where light-guide elementsoverlap in accordance with an embodiment of the invention;

FIG. 29 depicts a cross-section of two light-guide elements withtransition regions in accordance with an embodiment of the invention;

FIG. 30 graphically depicts the power output of each element shown inFIG. 29.

FIGS. 31-36 depict different methods and systems for overlappinglight-guide elements to form planar illumination areas with transitionregions, and the light-guide elements used therein, in accordance withembodiments of the invention;

FIGS. 37 and 38 depict cross-sections of planar illumination areas madewith light-guide elements having non-vertical sidewalls in theiroverlapping regions in accordance with embodiments of the invention;

FIG. 39A is an elevation of one embodiment of a planar illumination areaincluding a transparent filling material in accordance with anembodiment of the invention;

FIG. 39B is a perspective view of another embodiment of a planarillumination area including a transparent filling material in accordancewith an embodiment of the invention;

FIG. 40 depicts an LED sub-assembly in accordance with an embodiment ofthe invention;

FIGS. 41-43 depict a carrier platform and bare-die LEDs, a printedcircuit board, and an interface plate in accordance with embodiments ofthe invention;

FIG. 44 depicts a bottom view of the LED sub-assembly; and

FIG. 45 is a perspective view of a planar illumination unit inaccordance with embodiments of the invention.

DETAILED DESCRIPTION 1. Basic Architecture

Described herein are various embodiments of methods and systems forassembling a planar illumination area based on one or more discreteplanar illumination units, various embodiments of different types oflight-guide elements and LED sub-assemblies, and various embodiments ofmethods and systems for eliminating non-uniform “stitching” effectsbetween planar illumination unit tiles.

An advantage of the present invention, in various embodiments, is theability to utilize upward-emitting (e.g., Lambertian) light sources.FIG. 2 generically illustrates a monolithic structure 200 with anintegrated upward-emitting light source 202 embedded fully within alight guide 208. The light source 202 may have a Lambertian lightdistribution, and light 204 is substantially retained within the lightguide 208 for emission through a surface 210 thereof.

In general, the present invention utilizes modular light-guide elementsin which the light source is at least partially (and typically fully)embedded, facilitating the light retention and emission behavior shownin FIG. 2. The elements are tilable to facilitate uniformly illuminatingsurfaces of arbitrary size. A representative planar, tilableillumination unit 300 is illustrated in perspective in FIG. 3 andsectionally in FIG. 4. The illustrated planar illumination unit 300includes a pair of opposed LED sub-assembly modules 302 and alight-guide element 304. As explained more fully below, the light-guideelement 304 may include various regions—e.g., an in-coupling region 306,a concentration region 308, a propagation region 310, and anout-coupling region 312—that optimize capture, retention, and emissionof light. Each LED sub-assembly module 302 desirably includes an LEDlight source at least partially packaged within the in-coupling region306 of the light guide 304, as further described below.

Light may be emitted upward from the LED sources, which may beLambertian sources, into the in-coupling region 306 of the light guide304, in which case the light propagates in lateral directions (i.e., isconfined within the thickness of the light guide 304). The in-couplingand concentration regions 306, 308 of the light-guide element 304 ineffect gather the light from the light source and direct it, withminimal losses, toward the propagation region 310. In particular, theconcentration region 308 orients toward the out-coupling region 312 asubstantial fraction of multidirectional light received in thein-coupling region 306. Light from the concentration region 308traverses the propagation region 310 and advances to the light-emittingout-coupling region 312, reaching the interface 314 between theout-coupling region 312 and the propagation region 310 with adistribution suitable for the desired functionality of the light source.For example, in order to obtain a uniform light emission across theilluminating region, a uniform distribution of the light across theinterface 314 is preferred. It should be stressed that the interface 314is typically not a sharp boundary, but rather a gradient transitionestablished by, for example, a change in the density of scatteringparticles that occurs over a distinct zone. The same is true of theout-coupling sub-regions described below.

In the out-coupling region 312, the light is emitted from the lightguide, resulting in planar illumination with desired properties thatdepend on a particular application. For example, substantially uniformillumination over the entire area of the out-coupling region may bepreferred for back-lighting applications. (By “substantially uniform” ismeant no more than 10% variation in output intensity.) Monolithicwaveguides and methods for their manufacture are described in detail in,for example, the '110 application (titled “Waveguide Sheet and Methodfor Manufacturing the Same”) referenced above.

Exemplary properties exhibited by planar illumination systems accordingto embodiments of the present invention are shown in FIG. 5. The planarillumination system whose behavior is illustrated in FIG. 5 includes, asdiscrete light sources, two RGB midsize LED chips, and the thickness ofthe system is approximately 5 mm. These planar illumination systemsexhibit a brightness of approximately 2060 candelas per square meter(nits) with a brightness non-uniformity of only approximately ±6%. For(x, y) color coordinates of (0.254, 0.240), the color uniformity (Δx,Δy) may be approximately (±0.008, ±0.006).

With renewed reference to FIGS. 3 and 4, the depicted cylindrical shapeof the in-coupling region 306 is for illustrative purposes only, andother shapes may be used. For example, the in-coupling region 306 mayinclude two regions having annular or cylindrical profiles. One or morelight sources, for example, white, single color, red-green-blue (RGB),or infrared (IR) light sources, as well as either bare-die or packagedLEDs, may be mounted at the back of the in-coupling region 306. Theselight sources emit light that is coupled into the light guide throughthe in-coupling region 306, which alters the direction of the lightemitted by the LEDs to couple the light into the light-guide element304.

Once coupled into the light-guide element 304, the light may be emittedin all directions along the periphery of the in-coupling region 306.In-coupling is described in U.S. application Ser. No. 12/155,090, titled“Method and Device for Providing Circumferential Illumination,” filed onApr. 29, 2008, which is hereby incorporated by reference in itsentirety. For example, the in-coupling region 306 may take the form ofan optical funnel. The funnel receives light from one or morelight-emitting elements and transmits the light into the propagationregion 310. The funnel may take the form of a surface-emitting waveguideor a surface-emitting optical cavity that receives the light generatedby one or more LEDs through an entry surface, distributes it within aninternal volume, and emits it through an exit surface. To prevent orreduce optical losses, the in-coupling region 306 and/or concentrationregion 308 may include one or more reflectors (e.g., edge reflectors).

Because the out-coupling region 312 may be located on only one side ofthe in-coupling region 306, the light that is emitted from thein-coupling region 306 on a side 316 opposite the out-coupling region312 may be redirected toward the out-coupling region 312. Thisredirection may occur in the concentration region 308, where aconcentrator may direct the light toward the out-coupling region 312, asdescribed in greater detail below. In one embodiment, there is aconcentrator for each LED, such as, for example, two LEDs and twoconcentrators. In FIG. 3, one embodiment of a parabola-shapedconcentration region 308 is shown, but other shapes may be used. Thecenter of the parabola formed by the concentration region 308 may be thecenter of the in-coupling region 306.

The propagation region 310 allows the light transmitted from thein-coupling region 306 and concentration region 308 to propagate freelytoward the out-coupling region 312. In the out-coupling region 312, thelight is turned in an upward direction (as indicated at 318) from theplanar illumination unit 300 to the outside world. This light may thenilluminate a planar segment of a larger illumination surface formed bymultiple tiled illumination units 300 such as, for example, a surface inan LCD backlight application. The out-coupling region 312 is depicted assquare-shaped in FIG. 3, but, for illustrative purposes, is shown asthree rectangular sub-regions 320, 322, 324. These sub-regions 320, 322,324 are not separate regions, but instead show, in one embodiment, arepresentative distribution of dispersed particles within theout-coupling region 312. These particles facilitate emission of thelight by serving as scatterers, typically scattering optical radiationin more than one direction. When light is scattered by a particle suchthat the impinging angle is below the critical angle, no total internalreflection occurs and the scattered light is emitted through the surfaceof out-coupling region 312 along the direction 318.

The light-scattering particles may be beads, e.g., glass beads, or otherceramic particles, rubber particles, silica particles, particlesincluding or consisting essentially of inorganic materials such as BaSO₄or TiO₂, particles including or consisting essentially of a phosphormaterial (as further described below), and the like. In an embodiment,the light-scattering particles are substantially or even completelynon-phosphorescent. Such non-phosphorescent particles merely scatterlight without converting the wavelength of any of the light striking theparticles. The term “light-scattering particles” may also refer tonon-solid objects embedded in the waveguide material from which corestructure are made, provided that such objects are capable of scatteringthe light. Representative example of suitable non-solid objects include,without limitation, closed voids within the core structures, e.g., airbubbles, and/or droplets of liquid embedded within the core structures.The light-scattering particles may also be organic or biologicalparticles, such as, but not limited to, liposomes. In some embodiments,optical elements such as microlenses are utilized in conjunction with,or even instead of, light-scattering particles.

Typically, the particles are concentrated toward the center sub-region322 of the out-coupling region 312—i.e., the particle concentration inthe center sub-region 322 exceeds the concentration in the peripheralsub-regions 320, 324, but typically the particle-concentrationtransition among sub-regions is gradual rather than abrupt.

The same scattering material may be used for each region 306, 308, 310,312 but at different concentrations appropriate to the functions of thedifferent regions. The out-coupling region 312, for example, typicallycontains the greatest concentration of particles. The concentrationregion 308 may contain particles graded in concentration to direct lightto the propagation region 310, which typically contains no particles.

The concentration region 308 transfers the light that is coupled intothe light-guide element 304 so that it propagates toward the propagationregion 310. In addition, the concentration region 308 may enable thelight from the in-coupling region 306 to the out-coupling region 312 toachieve the required distribution of light intensity. The in-coupling306, concentration 308, and propagation 310 regions may be designed toevenly distribute light at the entrance 314 to the out-coupling region312. In other words, a standard structure for emitting light from theout-coupling region 312 may enforce a uniform distribution of intensityat the entrance 314 to the out-coupling region 312.

FIG. 6 illustrates an exemplary portion of a light-guide element 600including a concentration region 602, an in-coupling region 604, and apropagation region 606 that increases the amount and uniformity of lightintensity at the interface 610 to the out-coupling region by advancingthe light toward the out-coupling region in a uniform manner. A side 608of the light-guide element in the concentration region 602 has aparabolic shape and/or a reflective coating. The center of the parabolaformed by the concentration region 602 may be the center of thein-coupling region 604. Light that enters the light-guide element 600 atthe in-coupling region 604 may spread in all directions in thelight-guide element 600. The angular spread, as indicated in the figure,is largely confined to the concentration region 602 and directed towardthe propagation region 606 due to total internal reflection at thesidewall 608. The critical angle may be, for example, 41.8 degrees, suchthat any injected light propagating toward the sidewall 608 at or belowthis angle will ultimately reach the entrance 610 of the propagationregion 606, via a single or multiple reflections from the sidewall. Atthe entrance 610 to the propagation region 606, the light intensity willbe a superposition of the light directly propagating to the propagationregion 606 from the in-coupling region 604 and the light reflected fromthe sidewall 608. Desirably, the light intensity at the entrance 610 ofthe propagation region 606 is substantially uniform. In this way, thelight may propagate through the propagation region 606 and reach theentrance to the out-coupling region with a uniform intensitydistribution.

Because the refractive index of air is about one, the light-guideelement 304 may be made using a waveguide material having a refractiveindex greater than one. Representative examples of materials suitablefor the light-guide element include 304, without limitation, TPU(aliphatic), which has a refractive index of about 1.50; TPU (aromatic),which has a refractive index of from about 1.58 to about 1.60; amorphousnylon such as the GRILAMID material supplied by EMS Grivory (e.g.,GRILAMID TR90), which has a refractive index of about 1.54; the TPX(PMP) material supplied by Mitsui, which has a refractive index of about1.46; PVDF, which has a refractive index of about 1.34; otherthermoplastic fluorocarbon polymers; the STYROLUX (UV stabilized)material marketed by BASF, which has a refractive index of about 1.58;polymethyl methacrylate (PMMA) with a refractive index of about 1.5; andpolycarbonate with a refractive index of about 1.5. As explained in the'090 application, the light-guide element 304 may consist of a single(core) layer or have a sandwich structure in which a core layer liesbetween opposed cladding layers. The thickness of the cladding layers(if present) is typically from about 10 μm to about 100 μm. Thethickness of the core layer may vary from approximately 400 μm toapproximately 1300 μm.

In various embodiments, the material from which the light-guide elements304 are formed is transparent, is at least somewhat flexible, possessesat least some elongation capability, and/or is capable of being producedin a thermoplastic process. Very flexible materials such as silicone maybe suitable, as well as less flexible materials such as PMMA orpolycarbonate. The degree to which the chosen material is capable ofbending may depend on the mode of assembling sets of elements into asurface. For example, some assembly procedures may require little or nobending. In other embodiments, the material is not inherently flexible;even a relatively stiff material, if thin enough, may exhibit sufficientmechanical flexibility to accommodate assembly as described herein. Thewaveguide elements may be manufactured by any suitable techniqueincluding, without limitation, co-extrusion, die cutting, co-injectionmolding, or melting together side-by-side in order to introduce bendsthat will facilitate assembly.

Each region 306, 308, 310, 312 of the light-guide element 304 mayinclude phosphorescent materials that change the wavelength of the lightstriking them to another wavelength, thereby, for example, altering thecolor of the light. In this manner, white light may be produced byaltering the wavelength of some of the light emitted from the lightsources. During propagation to the out-coupling region 312, portions ofthe light may be absorbed by the phosphorescent material, which thenemits light of a different wavelength. Light with different wavelengthsmay be collectively emitted by the out-coupling region, forming whitelight.

2. Light-Guide Element Configurations

FIG. 7 illustrates one embodiment of a two-source light-guide element700 in accordance with the present invention. The two-source light-guideelement 700 has an in-coupling region 702, a concentration region 704, apropagation region 706, and an out-coupling region 708. The out-couplingregion 708 may be square-shaped, as shown, thereby allowing a firstlight-guide element 700 to be tiled next to a second, similarlight-guide element 700 rotated by 90 degrees. In this way, and asdescribed below, the out-coupling region 708 of the first light-guideelement 700 may be positioned above the in-coupling region 702,concentration region 704, and propagation region 706 of the secondlight-guide element 700, thereby hiding those regions of the secondlight-guide element; the result of tiling in this fashion is to producea uniform illumination surface without dark regions. In alternativeembodiments, however, the out-coupling region 708 is rectangular.

FIG. 8 illustrates, in another embodiment, a single-source light-guideelement 800 that has only a single in-coupling region 802 for receivinglight from a single source. The single-source light-guide element 800also includes a concentration region 804, a propagation region 806, andan out-coupling region 808. Light propagates from the in-coupling region802 to the out-coupling region 808 in a single direction. As with thetwo-source light-guide element 700, assembly of a planar illuminationarea using single-source light-guide elements 800 does not require themto be bent (since they may be tiled in a manner that allows theout-coupling region 808 to occlude the concentration region 806 of anadjacent element). For square out-coupling regions 808, the number ofdiscrete light-guide element 800 needed for assembly of the tiled planarillumination area increases with the square of the increase in thesurface diagonal of the illumination surface. (Like the two-sourcelight-guide element 700, the out-coupling region 808 is, in variousembodiments, rectangular or square-shaped.)

Light-guide elements in accordance with the invention may have multiplelight sources arranged in a single side of the element, as illustratedin FIG. 9. The depicted asymmetric light-guide element 900 has the formof a stripe with a single out-coupling region 902 and a plurality ofadjacent in-coupling 904, propagation 906, and concentration regions908. As illustrated, the in-coupling regions are disposed on only oneside of the out-coupling region 902, and light therefore reaches theout-coupling region 902 from only one side rather than from two. Theasymmetric light-guide element 900 may be used to assemble a planarillumination area of any size. The out-coupling region 902 is typicallyrectangular, as illustrated, but may be square or any other shape.

In some embodiments, the light-guide elements are folded rather thanoverlapped in assembly. FIG. 10 shows, in one embodiment, a foldedtwo-source two-direction light-guide element 1000. The folded element1000 has the configuration of two-source light-guide element 700, but isfolded over on itself such that the two light sources, in-couplingregion 1002, concentration region 1004, and propagation region 1006 arehidden under the light-guide element's out-coupling region 1008. Thefolded two-source light-guide element 1000 may have a square-shapedout-coupling region 1008; the symmetry of the square shape is desirablein allowing the folded two-source light-guide elements 1000 to be tiledside-by-side. In one embodiment, each folded two-source light-guideelement 1000 is rotated 90 degrees with respect to a neighboring element1000 so that each folded side of one element 1000 abuts an unfolded sideof a neighboring element 1000. The number of square two-sourcelight-guide elements 1000 needed for a planar illumination area assemblyincreases with the square of the increase in the area diagonal.

FIG. 11 shows a folded single-source, two-direction light-guide element1100. This light-guide element is similar to the folded two-sourcelight-guide element 1000, except that only one light source is present,and the in-coupling regions 1102 overlap. Light propagates from thelight source to the out-coupling region 1104 from opposite directions,i.e., through the in-coupling regions 1102. The number of squareone-source light-guide elements 1100 needed for the planar illuminationarea assembly increases with the square of the increase in the areadiagonal.

In various embodiments, the two-source light-guide element 700,single-source light-guide element 800, asymmetric light-guide element900, folded two-source light-guide element 1000, and/or foldedone-source light-guide element 1100 may be modified to change theirproperties in accordance with functional requirements. For example, themanner in which the in-coupling, concentration, and propagation regionsmate with the out-coupling region may be modified. In one embodiment,light from a single source is coupled to an out-coupling region from twoor more directions, thereby enabling more efficient and uniformout-coupling of the light. Other modifications may be made as well, suchas changing the shape of the out-coupling region to be either square orrectangular. A square shape imparts rotational symmetry, which maysimplify assembly of the planar illumination, while a rectangular shapefacilitates assembly of a rectangular planar illumination of any desiredsize. In addition, the flexibility of a light-guide element may beadjusted to comply with a particular tiling or folding technique, whichmay require that a light-guide element be bent to hide a non-illuminatedarea of an adjacent light-guide element. A light-guide element may alsohave more than one light source. The size of the light-guide element maybe adjusted to change the total number of light-guide elements requiredto assemble a planar illumination area; for example, a single planarconfiguration may utilize elements having different sizes orconfigurations.

3. Light-Guide Element Tiling and Planar Illumination Area Assembly

In accordance with embodiments of the present invention, an area of alight-guide element that does not emit light may be occluded by (i.e.,hidden behind) an area of another light-guide element that does emitlight; in particular, in-coupling, concentration, and/or propagationregions may be hidden under an out-coupling region. For example, theout-coupling region may be coupled to an in-coupling region on adifferent light-guide element. Accordingly, a large, uniformlyilluminated surface may be built even though some areas of thelight-guide element used to create the surface do not emit light. Thesurface may be configured in a variety of shapes, including curvedshapes or spheres.

The planar illumination area may be used to provide substantiallyuniform illumination in a variety of applications. In one embodiment,the planar illumination area is used as a luminaire for lightingapplications. In another embodiment, the planar illumination area isused as a backlight unit for a display device, e.g., a liquid crystaldisplay (LCD). In this embodiment, the LCD includes a plurality ofpixels and is placed in front of the light-guide elements.

Each planar illumination unit may represent an independent unit thatproduces and/or transfers light. A planar illumination area may beassembled from planar illumination units according to any of varioussuitable assembly techniques, such as segment assembly, stripe assembly,tile assembly, or folded architecture assembly, each described in moredetail below.

Segment assembly is a technique wherein, for the light-guide elementsdescribed above, each light-guide element is simply placed face-up on asurface. The out-coupling regions of some light-guide elements arearranged to cover the in-coupling regions of other light-guide elementspreviously put in place. For some light-guide elements, the out-couplingregions of previously placed light-guide elements are lifted so that thein-coupling regions of new light-guide elements can be slippedunderneath. This lifting step may require that at least some of thelight-guide elements exhibit sufficient flexibility to facilitatelifting. In one embodiment, each light-guide element has a rectangularout-coupling region that hides zero, one, or two light sources.

FIG. 12 illustrates an example of the segment assembly technique whereina planar illumination area 1200 is constructed from nine discretelight-guide elements 1202, which correspond to element 700 (but may be,in various embodiments, any of the light-guide elements describedabove). The structure 1200 emits light only from the square orrectangular out-coupling regions 1204 of the light-guide elements 1202.The in-coupling, concentration, and propagation regions 1206, 1208, and1210 of the light-guide elements 1202 do not emit light.

FIG. 13 illustrates a portion 1300 of the planar illumination area 1200in greater detail. An in-coupling region 1206 of first light-guideelement 1202 is hidden under a light-emitting out-coupling region 1204′of a second light-guide element 1202′. The out-coupling region 1204″ ofan adjacent element receives light from another in-coupling region 1206′of element 1202′.

FIG. 14 illustrates another segment assembly technique for constructinga large planar illumination area from discrete light-guide elements1402. In this example, nine light-guide element 1402 are assembled inthe manner shown into a three-by-three grid 1405 such that thenon-light-emitting portions of each light-guide element 1402 are hiddenbehind light-emitting portions of an adjacent light-guide element 1402.The integration principle illustrated in FIG. 14 may be applied toplanar illumination areas of arbitrary size, shape, and grid number.

Another light-source element configuration is illustrated in FIG. 15.The symmetric stripe element 500 represents a daisy-chaining oflight-guide elements 1502 such that each light-guide element 1502 sharesa light source with a neighboring light-guide element 1502. As a result,in-coupling regions 1504 of neighboring light-guide elements 1504overlap, and a stripe of N light-guide elements requires only N+1 lightsources. Each out-coupling region 1506, however, receives light from twodirections propagated from in-coupling regions 1504. The out-couplingregion 1506 may be square or rectangular. The light-guide elements 1502may need to be bent to assemble a planar illumination area with stripes1500. The number of discrete light-guide elements needed for the planarillumination area assembly may increase linearly with an increase in thearea diagonal.

Referring to FIGS. 16A and 16B, a planar illumination area 1600 may beassembled from stripes 1500 by arranging a first set of stripes 1500adjacently, and then weaving a second set of adjacent stripesperpendicularly through the first set of stripes. The over-and-underweaving is carried out so as to place an out-coupling region 1506 overeach in-coupling region 1504 and its associated concentration andpropagation regions. This procedure generally requires that thelight-guide elements 1502 exhibit some flexibility to permitinterweaving to take place.

FIGS. 17A and 17B show an exemplary planar illumination area 1700assembled using asymmetric light-guide elements 1702, which correspondto the elements 900 shown in FIG. 9. A first asymmetric light-guideelement 1702 is placed at the end edge of the illumination area. Asecond asymmetric light-guide element 1704 is placed next to the firstasymmetric light-guide element 1702 such that the out-coupling region1706 of the second asymmetric light-guide element 1704 covers thein-coupling regions 1708 of the asymmetric light-guide element 1702.Other asymmetric light-guide elements are added in the same fashion.This assembly technique does not require element flexibility becauseeach tile may be pre-formed to a desired form factor, and thus theasymmetric light-guide elements 1702 need not be bent. In anotherembodiment, single-source light-guide elements (corresponding to theelements 800 shown in FIG. 8) are placed adjacent to each other to forma group of elements similar to an asymmetric light-guide element 1702,and then this group of elements is used to form a structurecorresponding to the planar illumination area 1700.

FIG. 18 shows how a planar illumination area 1800 may be formed fromfolded two-source light-guide elements 1802 (corresponding to theelements 1000 shown in FIG. 10), which are simply tiled adjacently. Theplanar illumination area 1800 may also formed from folded one-sourcelight-guide elements (corresponding to the elements 1100 shown in FIG.11). The folded light-guide elements 1802 do not require hiding onelight-guide element behind another adjacent light-guide element becausethe out-coupling region of each folded light-guide element hides thein-coupling region of that light-guide element.

FIG. 19 shows a planar illumination area 1900 assembled using amultiple-light-source light-guide element 1902 (similar to themultiple-light-source element 900 shown in FIG. 9) that has itsin-coupling, concentration, and propagation regions folded underneathits out-coupling region. Because the out-coupling region of one foldedmultiple-light-source light-guide element 1902 need not be used to hidethe in-coupling, concentration, and propagation regions of an adjacentfolded multiple-light-source light-guide element 1904, the element 1902can simply be tiled adjacently; it is not necessary to bend the elements1902 to achieve planar assembly.

4. Stitching

A planar illumination area assembled from a plurality of light-guideelements as discussed above may emit non-uniform light at the boundaryregions, or “stitches,” between tiles.

There are several reasons why the stitches may emit non-uniform light.For example, the non-uniform light may be due to the configuration ofthe light-guide elements, stray light in the system, and/or roughness orroundness in a sidewall of a light-guide element owing to, for example,the light-guide elements themselves or their method of assembly. Thestructure of a planar illumination area that places each light-guideelement perpendicular to an adjacent light-guide element may create aproblem of uniformity in the borders of the light-guide elements due tothe positioning of the axis of the progress of the light between theadjacent tiles. The direction of the light emission from the tile in theout-coupling region may be similar to the direction of the progress ofthe light in the light-guide. When the tiles are positioned next to oneanother, a lack of uniformity may be created due to the non-continuityof the direction of the light emission between the tiles.

The non-uniform light may also be due to stray light in the system. FIG.20 illustrates a cross-section of a planar illumination area 2000 inwhich one light-guide element 2002 is laid on the surface of an adjacentlight-guide element 2004. This configuration may allow stray light 2010to pass from an in-coupling region 2008 of the first light-guideelement, between the two light-guide elements 2002, 2004, and then toemerge on the outside 2010 of the planar illumination area 2000.

In addition, as seen in the structure 2100 of FIG. 21, light 2102emitted from a lower light-guide element 2104 close to the edge of anupper light-guide element 2106 may meet and be reflected from a sidewall2108 of the upper light-guide element 2106. The original trajectory 2110of the light 2102 may thus be changed to the reflected path 2112. Thus,the sidewall 2108 of the upper light-guide element 2106 may create anon-uniform light pattern near it because it reflects emitted light 2102away from it.

Non-uniform light may also arise due to roughness and/or roundness ofthe sidewall of a light-guide element. FIG. 22 illustrates a structure2200 in which two adjacent light-guide elements 2202, 2204 are separatedby a distance d because of, for example, imperfections in the sidewalls2206 of the light-guide elements 2202, 2204. The gap 2208 between thelight-guide elements 2202, 2204 may also create a gap in thedistribution of emitted light 2210.

In various embodiments, through judicious placement and/or configurationof the light-guide elements, the amount of non-uniform light emitted atthe borders of the light-guide elements may be reduced. In addition, astructure may be added to a planar illumination area that createsblurring and conceals the visibility of the borders between thelight-guide elements.

In one embodiment, the walls of a light-guide element are modified toreduce the light emitted therefrom, and thereby reduce the non-uniformlight emitted at the borders between light-guide elements. For example,the walls of the light-guide element may be covered in a material thatabsorbs or reflects light, but does not prevent the emission ofintensified light from the end area. This diffuses at least part of thelight hitting the sidewall of the light-guide element in manydirections, and the light is emitted from the upper or lower surface ofthe light-guide. In another embodiment, the wall of the light-guideelement is polished to a tolerance of approximately 20 nmroot-mean-square or 150 nm peak-to-peak so that the light incident onthe sidewall of the light-guide element may be reflected or refractedinstead of diffused. In another embodiment, the wall of the light-guideelement is polished to a tolerance less than approximately 600 nmpeak-to-peak. If the light is refracted, it may pass through thepropagating light-guide element and enter a neighboring light-guideelement, where it may be emitted or again refracted. If the light isreflected, it may continue to propagate in the original light-guideelement.

In another embodiment, the shape of a sidewall of a light-guide elementmay be modified to affect the emission of light. FIG. 23 shows a portionof a light-guide element 2300 wherein the junction 2302, where thesidewall 2304 of the light-guide element meets a surface 2306 of thelight-guide element 2300, is curved. The curved area 2304 changes theangle of incidence of the light 2308 striking it, thereby permitting thelight 2308 to be refracted out of the light-guide element 2300. In arelated embodiment, light may also be emitted from a polished sidewallof a light-guide element if the light striking a portion of the sidewallstrikes with an appropriate angle in relation to the critical incidentangle.

In another embodiment, as shown in FIG. 24, the sidewall 2402 of thelight-guide element 2400 creates a right angle with respect to the upperand lower surfaces 2404, 2406 of the light-guide element 2400. The angleof the sidewall 2402 of the light-guide element 2400, alone or incombination with the polishing of the sidewall 2402, causes light 2408reaching the sidewall 2402 to be reflected, rather than being emittedfrom the light-guide element 2400.

A diffusive sheet may be used to reduce non-uniform light emitted fromthe borders of a light-guide element. Light is thereby emitted at a wideangle from the surface of a light-guide element near the border and in atransverse direction compared to the direction of the border line.Coupling two light-guide elements that emit in this manner blurs thevisibility of the border line with the help of a transparent diffusivesheet having a small diffusion value, such as, for example, 10-20%diffusive direction transmission and 80-70% reserve directiontransmission.

For example, as shown in FIG. 25, a planar illumination area 2500 may becovered by a transparent diffusive sheet. In this example, eachlight-guide element 2502 is sized 82 mm by 63 mm, and is separated froma neighboring light-guide element by 6 mm. The distance of the highestdiffuser from the illuminated surface is 4.5 mm. The non-uniformity ofthe stitch at the center of the black rectangle of FIG. 26 may besimulated using the calculation:

${{{Non}\text{-}{{Uniformity}\;\lbrack\%\rbrack}} = {\pm \frac{{Max} - {Min}}{{Max} + {Min}}}},$which, when applied to the planar illumination area 2500, indicates thatthe non-uniformity in light emission without using a diffuser is ±22%,while the non-uniformity with a diffuser in place is ±7%.

Emission of light at a wide angle may enable blurring of the borderlines between light-guide elements joined together along one axis andlaid over one another along a perpendicular axis. Light may be emitted awide angle in a direction perpendicular to the border line betweenlight-guide elements, and at a narrow angle in a direction parallel tothe border line, and a diffuser sheet may be placed over the light-guideelements. This structure increases the brightness of the illuminatingsurface, which may be useful for, for example, backlight unit (BLU)applications in which a brightness enhancement film (“BEF”) sheet isused to reduce the angle of the emitted light and obtain greaterbrightness.

There may be a lack of symmetry in the range of the light emission anglein the two axes. For BLU applications, the lack of symmetry may besuitable for the emission of wide angle light to be in the direction ofthe horizontal axis and the narrow angle light in the direction of thevertical axis. In one embodiment, shown in FIG. 26, a BLU 2200 includeslight-guide elements 2602 tiled next to one another in the direction ofthe horizontal axis 2604 and laid on top of one another in the directionof the vertical axis 2606. In another embodiment, the propagationdirection of light in a light-guide element 2602 is continuous withrespect to another aligned light-guide element, and mixing in thepropagation direction of the light is thereby reduced.

In another embodiment, shown in FIG. 27, the mixing of the lightdirections may be balanced by a suitable arrangement of optical prismsheets 2702, such as BEF sheets, that are placed above a planarillumination area 2704 to form a composite structure 2700. The directionof the optical prism sheets 2702 is generally aligned in the propagationdirection of the light in the light-guide elements 2706, and eachoptical prism sheet 2702 overlies the out-coupling region 2708 of alight-guide element 2706, separated by a gap 2710.

With reference to the structure 2800 shown in FIG. 28, a light-absorbingsurface 2802 may be placed in the region 2804 where two light-guideelements 2806, 2808 overlap to reduce the amount of light escapingbetween them, as explained with reference to FIG. 20. Thelight-absorbing surface 2802 may be a prism optical foil, such as a BEFsheet, that reduces the exit angles of this light, or, alternatively,bends the light back into the light-guide elements 2806, 2808 byallowing it to be coupled and spread inside. The light may thus berecycled so that it joins the light spreading in the light-guideelements 2806, 2808. When the stray light is at an obtuse angle relativeto the perpendicular direction of the light-guide elements 2806, 2808,more of the light may be recovered by the light-absorbing element 2802.

Adjacent light-guide elements may be overlapped to reduce a sharpcontrast between the illumination of the light-guide elements. FIG. 29shows a structure 2900 in which a first light-guide element 2902 is laidon top of an adjacent light-guide element 2904. The area of lightemission of the upper light-guide element 2902 covers not only thein-coupling, concentration, and propagation regions of the lowerlight-guide element 2904, but also a portion of its out-coupling region.This configuration allows the creation of a transition or overlap region2906 between the light-guide elements 2902, 2904. In one embodiment, anedge 2908 of the upper light-guide element 2902 forms a non-straightline. Light is transmitted from the out-coupling area of the lowerlight-guide element 2904 through the upper light-guide element 2902 tocreate a gradual change in the strength of the light across thetransition area 2906 between the light-guide elements 2902, 2904. Thisgradual change may be created by gradually decreasing the density oflight-scattering elements (e.g., particles as described above) 2910within each of the light-guide elements 2902, 2904 in the transitionregion 2906. FIG. 30 illustrates how the output power of each of thelight-guide elements 2902, 2904 gradually decreases within thetransition zone 2906. The sum 3000 of the output power of bothlight-guide elements 2902, 2904, however, should be approximatelyconstant (i.e., uniform) across the area of the transition zone 2906. Inone embodiment, the output power between the transition region 2906 andthe non-overlapping out-coupling regions of the light-guide elements2902, 2904 is substantially uniform, i.e., differs by no more than 10%.

Another planar illumination area 3100 made from overlapping light-guideelements 3102 is shown in FIG. 31. In this embodiment, the light-guideelements 3102 are tiled in a graded manner in one direction 3104 andplaced tightly together in the other direction 3106. FIGS. 32A and 32Bshow a top and bottom views 3200, 3202, respectively, of a light-guideelement 3204 that may be used in the structure 3100, and FIG. 33 shows aside view 3300. Transition regions 3206 exist on both sides of anout-coupling region 3208. Each light-guide element 3204 also features abottom reflector 3210, a light source 3212, and a transparent region3214. In this configuration, where the out-coupling region of an elementunderlies the out-coupling region of another element, it is transparentso as not to augment the light emitted from the overlying out-couplingregion.

In another embodiment, shown in FIG. 34, a planar illumination area 3400may be constructed by overlapping a series of light-guide elements 3402in two directions 3404, 3406. With reference to FIGS. 35 and 36, atransition region 3502 surrounds the out-coupling region 3504 on allfour sides thereof, as shown by the top view 3500 and bottom view 3600of a light-guide element 3508, allowing four other tiles to overlap, orbe overlapped by, all four sides of the transition region 3502. Anout-coupling region 3504, light source 3506, bottom reflector 3602, andtransparent region 3604 are also shown. The transition region 3502 maybe transparent on two or four sides, for example, depending on thecharacteristics of the transition regions of adjacent tiles; theobjective, once again, is to retain a constant light output across theoverlapping regions.

A sidewall of a first light-guide element may be formed such thatoverlapping the first light-guide element with a second does not cause alack of uniformity in the height of the formed planar illumination area.For example, as shown with respect to the planar illumination area 3700in FIG. 37, a first sidewall 3702 of a first light-guide element 3704may be non-vertical, and a second sidewall 3706 of an adjacent, secondlight-guide element 3708 may be non-vertical and complementary to thefirst sidewall 3706. The two light-guide elements 3704, 3708 overlap ina region 3710 that includes the non-vertical sidewalls 3702, 3706without a variation in the height h of the planar illumination area3700. In the alternative embodiment 3800 shown in FIG. 38, a sidewall3802 of a first light-guide element 3804 may be curved to fit into thecurvature 3806 of an adjacent, second light-guide element 3808.

FIGS. 39A-B illustrate, in alternative embodiments, side views of twoplanar illumination areas 3900, 3902 that include a transparent fillingmaterial 3904. The planar illumination area 3900 uses the transparentfilling material 3904 to reduce irregularities in the height of the area3900 produced by overlapping light-guide elements 3906. The refractiveindex of the transparent filling material 3904 preferably matches therefractive index of the light-guide elements 3906. In addition, use ofthe transparent filling material 3904 creates a flat and smoothillumination surface 3908. In an alternative embodiment, the planarillumination area 3902 includes transparent filling material 3904 in thespace between the light-guide elements 3906, and this transparentfilling material 3904 preferably has a refractive index that matches therefractive index of the light-guide elements 3906.

Utilization of a tile structure with a polished wall, as describedabove, in connection with the planar illumination area 3902 may helpcreate continuity between each light-guide element and its neighbor,allowing the light to spread between neighboring tiles. The merging ofthe light between the neighboring tiles desirably creates continuous andmonotonic change in the intensity of the light between the two sides ofthe stitch line without the need for an overlapping structure asdescribed above.

5. Led Sub-Assembly

In various embodiments of the present invention, an LED sub-assembly isattached to a light-guide element. The LED sub-assembly functions as aplatform for at least one light source and provides electrical andmechanical connectivity to the light-guide element. FIG. 40 illustratesan exemplary embodiment of an LED sub-assembly 4000, including a carrierplatform 4002, LED bare-die chips 4004, a printed circuit board (“PCB”)4006, and an interface plate 4008. These components are shown in greaterdetail in FIGS. 41-44. In other embodiments, the LED bare-die chips 4004may be replaced with packaged LED, RGB, or white light sources. Thelight sources may be either side-emitting or top-emitting (i.e.,Lambertian) sources.

FIG. 41 illustrates a structure 4100 including a carrier platform 4002suitable for supporting one or more light sources. The light sources maybe, for example, bare-die LED chips 4004. The carrier platform 4002 maybe any platform used for the assembly of LEDs, and, in one embodiment,exhibits good thermal conductivity. The carrier platform 4002 maymechanically support the LED bare-die chips 4004, enable heatdissipation from the LED bare-die chips 4004 by thermal conduction, andprovide electrical connectivity to the LED bare-die chips 4004.

FIGS. 42A and 42B illustrate, in one embodiment, top and bottom views,respectively, of the printed circuit board 4006. The carrier platform4002 with the LED bare-die chips 4004 may be mounted on the printedcircuit board 4006 via a connector. The printed circuit board 4006includes a contour electrical interface to supply electrical current tothe light sources. The printed circuit board 4006 also mechanicallysupports the carrier platform 4002 and is in thermal contact therewith,thus enhancing heat dissipation from the light sources.

FIG. 43 illustrates one embodiment of the interface plate 4008, whichprovides mechanical connectivity and support to an illumination source.The interface plate mechanically connects the entire LED sub-assembly4000 to a light-guide element. Further, it may enable mechanicalconnection of a planar illumination source to the required applicationstructure. It may also assist thermal dissipation by providing thermalconnectivity between the planar illumination source and the applicationstructure.

FIG. 44 illustrates a bottom view of the LED sub-assembly 4000, in whichan electrical interface 4402, mechanical interface 4404, and heatconduction interface 4406 are visible. FIG. 45 shows the LEDsub-assembly 4000 assembled together with a light-guide element 4502 toform a planar illumination source 4500. The LED bare-die chip 4004,mounted on the carrier platform 4002, may be placed in a suitable socketformed by the joining of the LED sub-assembly 4000 and the in-couplingregion 4504 of the light-guide element 4502. The light emitted from aLED bare-die chip 4004 is coupled to the in-coupling region 4504 of thelight-guide element 4502.

In one embodiment, the LED bare-die chip 4004 is placed in the LEDsub-assembly 4000, which is then attached to the light-guide element4502 following other assembly steps that require high temperatures(e.g., higher than approximately 85° C.) that may damage the polymers inthe light-guide element. Any gaps between the in-coupling region 4504 ofthe light-guide element 4502 and the carrier platform 4002 may be filledwith a suitable filler material. The filler material can tolerate theoperating temperatures of the LED (e.g., lower than approximately 150°C., or even lower than approximately 70° C.), but may not be capable oftolerating the higher temperatures required for assembly (e.g.,soldering, at approximately 250° C.) of the LED sub-assembly 4000. Thefiller material generally fills the LED socket and covers the surface ofthe LED die and any wire bonds connected thereto. Examples of suitablefiller materials include UV-curable adhesives such as LIGHT WELD 9620,available from Dymax Corporation of Torrington, Conn., and encapsulationgels such as LS-3249 and LS-3252, available from NuSil Technology LLC ofWareham, Mass.

The LED bare-die chip 4004 may be coupled directly into the light-guideelement 4502 using an intermediary material with suitable optical andmechanical characteristics. This intermediary material may be all or aportion of an encapsulation structure disposed over the LED bare-diechip 4004. The form of the encapsulation is dictated by the shape andrefractive-index requirements of the optical interface with thelight-guide element 4502. If an encapsulation element is used, the spacebetween the walls of the socket in the light-guide element 4502 and theexternal surface of the encapsulation structure may be filled withoptical glue with suitable optical and mechanical characteristics.

The terms and expressions employed herein are used as terms andexpressions of description and not of limitation, and there is nointention, in the use of such terms and expressions, of excluding anyequivalents of the features shown and described or portions thereof. Inaddition, having described certain embodiments of the invention, it willbe apparent to those of ordinary skill in the art that other embodimentsincorporating the concepts disclosed herein may be used withoutdeparting from the spirit and scope of the invention. Accordingly, thedescribed embodiments are to be considered in all respects as onlyillustrative and not restrictive.

1. A planar illumination area comprising: a first discrete light-guideelement comprising (i) a first light source emitting light therein, (ii)first and second adjacent non-parallel sidewalls spanning top and bottomsurfaces, and (iii) a first out-coupling region in the top surface; anda second discrete light-guide element in contact with the first discretelight-guide element and comprising (i) a second light source, differentfrom the first light source, emitting light therein, (ii) first andsecond adjacent non-parallel sidewalls spanning top and bottom surfaces,and (iii) a second out-coupling region in the top surface, only aportion of the second out-coupling region, including at least onesidewall of the second light-guide element, being disposed over andoverlapping only a portion of the first out-coupling region including atleast one sidewall of the first light-guide element, to define anoverlapping region having an area, wherein (i) a light output power ofthe first out-coupling region within the overlapping region is less thana light output power of the first out-coupling region outside theoverlapping region and (ii) a light output power of the secondout-coupling region within the overlapping region is less than a lightoutput power of the second out-coupling region outside the overlappingregion, whereby the overlapping region emits a substantially uniformlight output power over the area.
 2. The planar illumination area ofclaim 1, wherein the light output power of the overlapping regiondiffers from the light output power of the first and second out-couplingregions outside the overlapping region by no more than 10%.
 3. Theplanar illumination area of claim 2, wherein the change in light outputpower between the overlapping region and the first and secondout-coupling regions outside the overlapping region is gradual.
 4. Theplanar illumination area of claim 1, wherein the first out-couplingregion comprises light-scattering elements therein, a density of thelight-scattering elements disposed within the first out-coupling regionoutside the overlapping region being greater than a density of thelight-scattering elements disposed within the first out-coupling regioninside the overlapping region.
 5. The planar illumination area of claim4, wherein the density of the light-scattering elements inside theoverlapping region decreases along one direction.
 6. The planarillumination area of claim 1, wherein the second out-coupling region istransparent in the overlapping region so as not to augment light emittedfrom the first out-coupling region.
 7. The planar illumination area ofclaim 6, wherein, in the overlapping region, light emitted from thefirst out-coupling region passes through the second out-coupling region.8. The planar illumination area of claim 1, further comprising a thirddiscrete light-guide element comprising a third out-coupling region inits top surface, only a portion of the third out-coupling regionoverlapping only a second portion of the second out-coupling region todefine a second overlapping region different from the overlapping regionand having a second area, wherein the second overlapping region emits asubstantially uniform light output power over the second area.
 9. Theplanar illumination area of claim 1, wherein at least a portion of thefirst sidewall of the first light-guide element (i) is non-vertical and(ii) forms a non-right angle with at least one of the top or bottomsurfaces of the first light-guide element, and at least a portion of thefirst sidewall of the second light-guide element (i) is non-vertical,(ii) forms a non-right angle with at least one of the top or bottomsurfaces of the second light-guide element, and (iii) is complementaryto the first sidewall of the first light-guide element, the firstsidewalls of the first and second light-guide element being disposedbeneath a top surface of the second light-guide element in theoverlapping region.
 10. The planar illumination area of claim 9, whereina top surface of the planar illumination area comprises the top surfacesof the first and second light-guide elements and is substantially flat.11. The planar illumination area of claim 1, further comprisingadditional discrete light-guide elements forming, with the first andsecond light-guide elements, an array extending in first and secondnon-parallel directions, wherein the light-guide elements overlap in thefirst direction and do not overlap in the second direction.
 12. Theplanar illumination area of claim 1, further comprising additionaldiscrete light-guide elements forming, with the first and secondlight-guide elements, an array extending in first and secondnon-parallel directions, wherein the light-guide elements overlap in thefirst and second directions.
 13. The planar illumination area of claim1, wherein the overlapping region comprises a light-absorbing material.14. The planar illumination area of claim 1, wherein at least a portionof a sidewall of the first light-guide element comprises a reflectivecoating.
 15. The planar illumination area of claim 1, wherein at least aportion of a sidewall of the second light-guide element comprises areflective coating.
 16. The planar illumination area of claim 1, whereinonly a portion of a top surface of at least one of the first light-guideelement or the second light-guide element is a light-absorbing surface.17. A method of forming a planar illumination area, the methodcomprising: providing a first discrete light-guide element comprising(i) a first light source emitting light therein, (ii) first and secondnon-parallel sidewalls spanning top and bottom surfaces, and (iii) afirst out-coupling region in the top surface; and disposing beneath andoverlapping only a portion of the first light-guide element with asecond discrete light-guide element comprising (i) a second lightsource, different from the first light source, emitting light therein,(ii) first and second non-parallel sidewalls spanning top and bottomsurfaces, and (iii) a second out-coupling region in the top surface,thereby forming an overlapping region comprising only a portion of thefirst out-coupling region including at least one sidewall of the firstlight-guide element and only a portion of the second out-coupling regionincluding at least one sidewall of the second light-guide element,wherein (i) a light output power of the first out-coupling region withinthe overlapping region is less than a light output power of the firstout-coupling region outside the overlapping region and (ii) a lightoutput power of the second out-coupling region within the overlappingregion is less than a light output power of the second out-couplingregion outside the overlapping region, whereby the overlapping regionemits a substantially uniform light output power.
 18. A planarillumination area comprising: a first discrete light-guide elementcomprising a polished sidewall disposed between a top surface and abottom surface of the first light-guide element; and a second discretelight-guide element comprising a polished sidewall disposed between atop surface and a bottom surface of the second light-guide element, thepolished sidewall of the second light-guide element being adjacent tothe polished sidewall of the first light-guide element, wherein thesecond light-guide element receives light from the first light-guideelement through the polished sidewalls, and light is refracted as ittravels from the first light-guide element to the second light-guideelement.
 19. The planar illumination area of claim 18, furthercomprising an index-matching material located between the sidewalls, anindex of refraction of the index-matching material being approximatelyequal to an index of refraction of at least one of the first light-guideelement or the second light-guide element.
 20. The planar illuminationarea of claim 18, wherein the first and second polished sidewalls arepolished to a tolerance less than approximately 600 nm peak-to-peak. 21.A planar illumination area comprising: a first discrete light-guideelement comprising a sidewall disposed between a light-emitting topsurface and a bottom surface of the first light-guide element; a seconddiscrete light-guide element comprising a sidewall disposed between alight-emitting top surface and a bottom surface of the secondlight-guide element, the first and second light-guide elements beingbutted together or only partially overlapping such that theirlight-emitting top surfaces collectively form a single continuousillumination surface including at least one of the sidewalls; and anindex-matching material covering the sidewalls, an index of refractionof the index-matching material being approximately equal to an index ofrefraction of at least one of the first light-guide element or thesecond light-guide element, wherein light travels between the first andsecond light-guide elements, and, in a direction of travel from thefirst light-guide element to the second light-guide element, anintensity of light emitted by the continuous illumination surfacechanges continuously and monotonically notwithstanding the includedsidewalls.
 22. The planar illumination area of claim 21, wherein thesidewall of the second light-guide element is adjacent to the sidewallof the first light-guide element and the index-matching material islocated between the sidewalls.
 23. The planar illumination of claim 21,wherein the first and second light-guide elements only partiallyoverlap, and the index-matching material is disposed over and in directcontact with the first and second light-guide elements to create asmooth surface thereover.